Many optical devices, such as some microscopes, telescopes and night vision devices, are viewed with binocular vision but have only a single objective lens arrangement. When a single source image is viewed with binocular vision, the light from the image must obviously be divided into separate corresponding optical outputs that can be viewed simultaneously. Traditionally, the division of a single source image into two corresponding binocular images has been accomplished by utilizing a collimator lens assembly. Traditionally, a collimator is an optical apparatus for producing parallel rays of light. A rudimentary form consists of a converging lens, at one focal point of which is placed a small source of light. The source of light is usually a pinhole or narrow slit from which the light is caused to emanate. Rays diverging from this focal point emerge from the objective lens in a parallel beam, in accordance to the definition of "focal point". The slit or other source, as viewed through the collimator, appear as though located at an infinite distance. The collimator lens assembly therefore re-images the source image at a substantially infinite conjugate. Consequently, the collimated light from the re-imaged source image can be split equally, without distortion or parallax, and directed into each of the eyepiece assemblies for viewing. The eyepiece assemblies are configured to receive collimated light from the collimator lens assembly and are sized and located to share lightbeam sections included within the collimator exit pupil.
Traditional collimator lens assemblies are usually formed from lenses that are symmetrically constructed about the longitudinal axis of the assembly. As such, the optical axes of the lenses, within the collimator lens assembly, are linearly aligned and rotationally symmetric about the longitudinal axis. It is well known that, in a linearly aligned, rotationally symmetric, lens arrangement, the best optical performance is usually achieved on light traveling down the optical axis of the lenses. Such an optical path is termed to be "on-axis" with the lens assembly. The optical performance of the lens assembly is reduced for light traveling "off-axis" in the field of view. The optical performance usually becomes progressively worse as light travels off-axis toward the edge of the field of view. Since many traditional collimator lens assemblies have a common optical axis for all of its component lenses, the collimator lens assembly has only one on-axis position centrally positioned within the single optical path.
Since such traditional collimator lens assemblies have only one centered pupil position, it is impossible to direct all the light passing through the center of the pupil position to both eyepieces simultaneously, when the light is split for binocular viewing. When the image produced by the collimator lens assembly is split, it is usually split evenly on either side of the center of the pupil position. Consequently, mostly decentered or "eccentric pupil" light is directed into each eyepiece and the optical performance of the collimator lens assembly may be sacrificed for the sake of performance balance with the eyepiece induced aberrations present during binocular viewing.
A typical prior art optical device that divides collimated light for binocular viewing is shown in FIG. 1. Other optical devices, that utilize split light from a collimator lens assembly in a decentered pupil and/or off-axis position, are exemplified in U.S. Pat. Nos. 3,781,560 to DeBurgh et al., 4,266,129 to Versteeg et al., 4,392,710 to Rogers and 4,463,252 to Brennan et al. All the above referenced patents referring to night vision optical devices.
Besides off-axis and/or decentered use of collimator lenses, single image source optical systems that employ binocular viewing have other disadvantages. Primary among the disadvantages is the formation of chromatic aberrations in the viewed image.
It is a well known phenomenon that the passage of light through an optical lens causes chromatic aberrations. Collimator lens assemblies include many different component lenses, thus many prior art collimator lens assemblies are constructed to reduce the chromatic aberrations by varying the material, shape and arrangement of each of the component lenses to compensate for the aberrations. Similarly, many multiple lensed magnifying or reducing optical assemblies also compensate for some chromatic aberrations by varying component lens construction. Prior art applications where a lens arrangement is designed to compensate for some chromatic aberrations are described in the below listed U.S. Pat. Nos.:
______________________________________ 5,011,272 to Nakayama et al. 4,397,520 to Neil 4,963,010 to Kikuchi 4,365,871 to Muchel 4,871,219 to Cooper 4,171,872 to Baker 4,641,927 to Prescott et al. 3,827,785 to Matsusbita et al. 4,435,041 to Torok et al. 3,637,295 to Matsumura et al. 4,411,488 to Neil 3,604,786 to Baker ______________________________________
However, the lenses forming the collimator lens assembly are not the only optical elements in the system that can cause chromatic aberrations. In binocular viewing two eyepiece lens assemblies are used; one for each eye. The eyepiece assemblies may have lenses to magnify an image or a simple lens arrangement to invert a previous inverted image for normal viewing. Often the chromatic aberrations of the eyepiece optics is not fully corrected; thus reducing the quality of the image viewed. In prior art applications the axial chromatic aberrations are often ignored as being insignificant or too difficult to correct with cost effective lenses. In classical aberration theory there exists a conventional aberration limitation to lens design performance called "axial chromatic aberration" or "axial color". This is defined as the degree to which red, green and blue light, or other long, nominal and short wavelengths, are focused at different locations along the optical axis. In applications such as night vision optics, the viewed image may be within a more restrictive wavelength range than blue to red light, but still may produce substantial chromatic aberrations that can cause a large reduction in the quality of the viewed image.
It is therefore a primary objective of the present invention to provide an optical assembly wherein a single image source is directed to two eyepieces for binocular viewing such that each eyepiece receives light from a decentered section of the exit pupil of a collimator assembly.
It is a further object of the present invention to provide a collimator assembly that corrects the image to compensate for, and reduce, axial chromatic aberrations that will undesirably distort the image due to optical elements of the eyepieces through which the image is viewed.